It's built using Catalyst, their technology for running iOS apps on MacOS, which was only launched with Catalina, so there's a good reason for this.

I recently got a new Macbook and was dreading having to use Catalina, having heard horror stories online, but actually my experience has been fine. A few security popups to click through when you first run a new app, but some of the screen shots I saw online of hundreds of them must have been fake, it's really not a problem. Having to right click and open unidentified apps can be annoying, but I'd rather do that and keep Gatekeeper on personally, rather than disabling Gatekeeper entirely.

It actually feels really solid so far (granted its a clean install on a new laptop) and I am really impressed with the Sidecar feature in particular. Of course, if you depend on 32-bit apps or drivers, or certain apps which are meant to be buggy (e.g. Mail referenced in another comment), you may want to hold off for a while!